● Covered by 1 source · 1 reportMedium impact1 negative
October Windows Server Updates Break Exchange Sign-in and Mail Flow
Key points
- October 2026 Windows Server updates cause Exchange issues.
- OWA sign-in and mail flow are affected.
- Problem due to Endpoint Mapper memory limit.
- Microsoft provided a registry workaround.
Windows Server Updates Cause Exchange Issues
Microsoft has confirmed that the October 2026 Windows Server security updates are leading to problems with Outlook on the web (OWA) sign-in and mail delivery for some Exchange Server systems. Affected users may find emails stuck in their Outbox or experience issues with mailbox replication.
Technical Details of the Problem
The root cause is a new default limit on memory used by Endpoint Mapper for Endpoint Lookup calls, introduced with the October 2026 update. For workloads like Exchange Server, the default limit of 512 simultaneous calls can be insufficient under heavy load, leading to errors such as "MapiExceptionMdbOffline" and Event ID 1006.
Microsoft clarified that these issues are not related to the October 2026 security updates specifically for Exchange Server, but rather impact any Exchange version running on affected Windows Server versions. The severity and speed of impact vary based on the individual Exchange server's load.
Temporary Workaround Provided
While Microsoft develops a permanent solution, it has released a temporary workaround. Administrators can modify a registry key on affected Exchange Server systems to increase the maximum number of simultaneous Endpoint Lookup calls. This involves running a specific command from an elevated Command Prompt and then rebooting the device.
The command to execute is: reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f. After restarting, administrators should monitor the Exchange environment and adjust the memory limit further if issues persist.
✨ This summary was generated by AI from the outlets' reporting listed below. It is not independently verified and may contain errors — check the original sources. How BrevFeed works →
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Reporting from
Microsoft confirmed that October 2026 Windows Server security updates are causing OWA sign-in and mail delivery issues on some Exchange Server systems. The problem stems from a new default memory limit for Endpoint Mapper, which can be insufficient under heavy Exchange Server load. Microsoft provided a temporary registry modification workaround while it works on a permanent fix.